The invention relates generally to gain control of photomultiplier tubes (PMTs) and more specifically concerns gain control of PMTs used in detecting differential absorption LIDAR returns.
The purpose of the invention is to change the gains of a PMT to accommodate differential laser signal returns produced by pulsed lasers with temporal separations of a few hundred microseconds operating at wavelengths ON and OFF the absorption lines of various trace gases. The gain of a PMT must be low during LASE to minimize saturation, increase to its maximum when the ON-LINE return is being received, and increase to a value below maximum a few hundred microseconds later when the stronger OFF-LINE return is being received.
The prior art involves designing the high voltage (-1000 to 3000 V) PMT divider network so that the focusing electrode voltage is near that of the cathode for minimum gain, and supplying a capacitive coupled externally produced 300 to 800 volt pulse-pair signal to drive the focusing electrode voltage close to that of the first dynode in order to obtain the required ON and OFF-LINE gains.
An alternative technique is to tie the odd-numbered dynodes to a voltage divider, offset the even-numbered dynodes about 30 volts from nominal divider voltages in order to reduce the gain, and use a 0 to -30 volt pulse-pair signal coupled through capacitors in order to drive all even-numbered dynodes back to their required ON and OFF-LINE gains.
Other techniques do not provide a means of setting the ON-LINE gain different from the OFF-LINE gain, and/or are not satisfactory for differential absorption type LIDAR returns separated by a few hundred microseconds.
A major disadvantage of the prior art is the requirement to capacitor-couple the externally produced pulse-pair control signals into the high-impedance high-voltage PMT divider network. The network does not completely recover during the few hundred microseconds between ON and OFF-LINE returns resulting in reduced accuracy of the differential measurements.
Other disadvantages are the bulky adjustable power supplies, high-voltage switching networks requiring increased maintenance costs and time, and the increased logic circuitry to produce two different amplitude PMT gain control pulses.
An object of this invention is to sequentially control the flow of electrons within PMTs using the focus electrodes, individual dynodes, and/or combinations of dynodes in order to change the gain to accommodate differential laser signal returns produced by pulsed lasers with temporal separations of a few hundred microseconds operating at wavelengths ON and OFF the absorption line of various trace gases.
Another object of this invention is to provide a PMT gain control that eliminates the requirement of capacitor coupling thereby providing increased accuracy of differential measurements.
A further object of this invention is to provide a PMT gain control which does not require bulky external adjustable power supplies with high-voltage switching.
Yet another object of this invention is to provide various preselect PMT gains using a control method that does not require different amplitude control signals.
A still further object of this invention is to provide a PMT gain control that can utilize fiber optic control and thereby minimize electromagnetic interference effects and reduce switching transients.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.